Recently, an amount of information required by information equipment and audio and video equipment is increasing. This causes attention to an information recording medium (hereinafter referred to merely as a “recording medium” or a “medium”) such as an optical disc which is advantageous in data accessibility, large-volume date storage and downsizing of equipment, and a recording density of the medium is increasing. For example, an optical head wherein a wavelength of laser beam is about 400 nm and a collecting lens for focusing the laser beam has a numerical aperture (NA) of 0.8 has been already used as a means for increasing the recording density of the optical disc. The use of this optical head makes it possible to record information of about 25 GB on a single-layer optical recording medium (which is the medium having a single recording layer) and record information of about 50 GB on the optical recording medium having two recording layers (see, for example, Patent Literature 1).
A structure and a production method of a conventional multilayer information recording medium described in Patent Literature 1 are described below with reference to FIGS. 5 and 6.
FIG. 5 is a cross-sectional view of the conventional multilayer information recording medium. This multilayer information recording medium consists of:
a first signal substrate 601 wherein an signal portion having pits and a groove and so on constructed by concavo-convex shape formed on one surface by transfer;
a first thin film layer 602 which is located on the concavo-convex shaped surface of the first signal substrate 601 on which surface is formed;
a second signal substrate 603 wherein a signal portion having the pits and the guide groove and so on constructed by the concavo-convex shape are formed by transfer on a surface opposite to a surface which is bonded to the first thin film layer 602;
a second thin film layer 604 which is located on the concavo-convex shaped surface of the second signal substrate 603 is formed; and
a transparent layer 605 which is formed so as to cover the second thin film layer 604. The first signal substrate 601 is formed of a resin material such as polycarbonate or polyolefin by injection compression molding or the like. During this molding, the signal portion such as pits and guide groove and so on are formed, on one side of the first signal substrate 601, as the concavo-convex shape by transfer. A thickness of the first signal substrate is about 1.1 mm.
Both of the first thin film layer 602 and the second thin film layer 604 include a recording film and a reflective film which are formed by a sputtering method or a vapor deposition method or the like on the signal portion-formed surfaces (signal surfaces) of the first signal substrate 601 and the second signal substrate 603. A material which exhibits an effective reflectance relative to the laser beam having a wavelength of about 400 nm is employed as the material for the reflective film. Such material is, for example, a metal material such as a silver alloy or aluminum.
The material for the recording film is selected depending on whether the recording medium is made a rewritable type or a write-once type. The recording film for the rewritable-type recording medium is formed selecting a material such that data can be recorded and erased two or more times. Specifically, a recording material such as GeSbTe or AgInSbTe is used. The recording film for the write-once-type recording medium is formed selecting a material which changes irreversibly such that the recording can be made only once. Specifically, TeOPd is a representative material.
The signal substrate 603 is formed by forming a layer using an ultraviolet curable resin by a spin coat method and then transferring the concavo-convex shape (the signal portion) such as the pits and the guide groove using a signal transfer substrate. The signal transfer substrate is a substrate wherein the concavo-convex shape such as the pits and the guide groove is formed on one side thereof similarly to the first signal substrate 601. Specifically, the signal transfer substrate is a substrate which is provided with a transfer surface having a signal surface wherein the concavo-convex shape corresponding to the signal portion to be formed on the second signal substrate 603 is formed. The second signal substrate 603 is formed by contacting the signal transfer substrate with the ultraviolet curable resin with the signal surface thereof facing the first signal substrate 601 followed by curing the ultraviolet curable resin and then peeling the signal transfer substrate.
The transparent layer 605 is made from a material which is transparent to a recording and reproduction light (that is, which has a high transmittance) and has a thickness of about 0.1 mm. A light curable resin or an adhesive such as a pressure-sensitive adhesive may be used as a material for the transparent layer 605. Specifically, the transparent layer 605 may be formed by, for example, applying an ultraviolet curable resin on the second thin film layer 604 by a spin coat method. The information is recorded on and reproduced from the multilayer information recording medium manufactured in this manner by allowing a recording and reproduction laser beam to enter from the transparent layer 605 side.
Cross-sectional views of the respective steps in the conventional production method of the multilayer information recording medium are shown in FIGS. 6(A) to (G). The conventional production method of multilayer information recording medium is described in detail with reference to these drawings.
Firstly, a first thin film layer 702 including a recording film and a reflective film is formed by a method such as sputtering or vapor deposition on a signal surface of a first signal substrate 701. The first signal substrate 701 is fixed to a rotatable table 703. The fixation is made by applying vacuum on the surface which is opposite to the surface where the first thin film layer 702 is formed (see FIG. 6(A)).
An ultraviolet curable resin 704 is applied to a desired position concentrically with the first signal substrate 701 (that is, in a circular pattern having a predetermined diameter) from a dispenser in order that a second signal substrate which is a resin layer (see FIG. 6(B)).
Next, the rotatable table 703 is rotated such that the ultraviolet curable resin 704 is spread toward an outer periphery into a film (that is, the application by the spin coat method is conducted) (see FIG. 6(C)). Further, extra resin and bubbles are removed by a centrifugal force exerted to the ultraviolet curable resin 704 during the rotation, which results in the formation of a thin layer. The thickness of the formed layer, that is, the second signal substrate 710 is adjusted to a desired thickness by arbitrarily selecting a viscosity of the ultraviolet curable resin 704, a rotation number, a rotation time and an atmosphere (a temperature and a humidity) during the rotation.
A signal transfer substrate 705 is overlaid on the thin-film ultraviolet curable resin 704 (see FIG. 6(D)). The signal transfer substrate 705 has a surface (signal surface) wherein the pits and the guide groove are formed as the concavo-convex shape similarly to the first signal substrate 701. The signal transfer substrate 705 is made of a material such as polycarbonate or polyolefin. The signal transfer substrate, 705 is overlaid such that the signal surface thereof and the signal surface of the first signal substrate 701 face each other. The overlaying step is preferably conducted under vacuum atmosphere in order to prevent the bubbles to be immixed between the signal transfer substrate 705 and the ultraviolet curable resin 704.
A multilayer structure 706 wherein the first signal substrate 701, the first thin film layer 702, the ultraviolet curable resin 704 and the signal transfer substrate 705 are integrated is irradiated with an ultraviolet ray which is applied from the signal transfer substrate 705 side in order that the ultraviolet curable resin 704 located between the two signal surfaces is cured (see FIG. 6(E)). The ultraviolet ray is applied by an ultraviolet ray irradiator 707. The reason why the ultraviolet ray is applied from the signal transfer substrate 705 is that the material, such as polycarbonate or polyolefin, which constitutes the signal transfer substrate 705 allows the ultraviolet ray to pass therethrough to some extent and to reach the ultraviolet ray to the ultraviolet curable resin 704.
After the ultraviolet curable resin 704 has been cured, the signal transfer substrate 705 is peeled at an interface between the ultraviolet curable resin 704 and the substrate 705, whereby the second signal substrate 710 is formed wherein the signal surface is formed by transfer (see FIG. 6(F)).
A second thin film layer 708 which includes the recording film and the reflective film is formed on the signal surface of the second signal substrate 710 by a method such as sputtering or vapor deposition. Finally, a transparent layer 709 which is almost transparent to the recording and reproduction light (that is, which has a high transmittance) is formed by, for example, spin-coating the ultraviolet curable resin followed by curing the resin by irradiation of the ultraviolet ray (see FIG. 6(G)).
As described above, in the conventional production method of the multilayer information recording medium, the second signal substrate is formed by a method wherein the ultraviolet curable resin is cured by being irradiated with the ultraviolet ray which has passed through the signal transfer substrate. For this reason, it is necessary to use the signal transfer substrate formed of a material which has a sufficiently high ultraviolet-ray transmittance (such as polycarbonate or polyolefin).